The present invention relates to motor controllers for use with material winders and more particularly to a fuzzy logic winder analyzer which facilitates inertia compensation and proportional-integral regulation to maintain desired tension of a material being wound by a winder.
A typical AC induction motor includes a rotor mounted for rotation inside a stator, a shaft integrally connected to the rotor and extending from the stator. The rotor includes a plurality of rotor windings and the stator includes a plurality of stator windings. To rotate the rotor, time varying voltages are applied across the stator windings causing a rotating magnetic stator field in the space including the rotor. The stator field induces (hence the term induction motor) a current in the rotor windings as the rotor windings "pass" through the stator field. The rotor current in turn generates a rotor field. The rotor and stator fields interact (e.g. attract) thereby causing a torque which rotates the rotor. Torque is a twisting force which is a function of rotor and stator field strengths and the proximity of the rotor field with respect to the stator field.
Motors are regulated by motor controllers. A motor controller usually includes an electronic processor which receives command signals indicating desired operating characteristics and generates stator voltages to drive the motor in a manner which will achieve the characteristics indicated by the command signals.
Motors are routinely used to drive material winders. A material winder is a machine used to wind thin sheet material (e.g. paper, sheet metal, sheet plastic, cloth, etc.) into rolls on spools for bulk storage and delivery. Winders are typically fed by material manufacturing machines which ideally provide sheet material at a constant line speed.
An extremely important consideration when winding material on a spool is material tension. If an ideal material tension is not maintained during winding, a resulting material roll may have any of a number of different defects. For example, if tension is to low, material may bunch on a spool causing unintended material overlap. If tension is far to low, material may even loop off the spool becoming tangled in winder hardware or becoming damaged or contaminated by contacting a floor or other proximal surface.
In addition, if tension is to high, material may rip. Ripping is a particularly important problem where the winding material is relatively delicate as in the case of most paper materials and some thin plastic materials. Moreover, if tension is to high material can become compressed or compacted on the roll as additional layers of material are added which generate a radially inward force on lower layers of material. In addition to changing the structural integrity of a material, compression can render material more susceptible to ripping when unwound for subsequent use.
To maintain a desired material tension winder motor controllers attempt to wind material on a spool at a rate which is identical to the line speed. Thus, if the line speed is 1000 feet per minute, the winder must rotate at a spindle speed such that 1000 feet per minute of material is wound. Unfortunately, the task of maintaining a constant desired command tension is frustrated by the fact that, while the line speed is ideally constant, in reality the line speed provided by material manufacturing machines often fluctuates.
To conform actual operating characteristics to command characteristics, many controllers include one or more feedback loops which provide feedback signals for comparison to command signals. For example, to maintain a desired tension, a command tension is provided to a motor controller. The controller is equipped with a sensor (e.g. a loadcell) which senses material tension and provides a tension feedback signal. The controller compares the feedback and command tension signals and generates a tension error signal. The error signal is provided to a proportional--integral (PI) regulator which generates a modified torque signal to eliminate the tension error.
A variety of different functional relationships may be implemented through the use of a PI regulator which provides a generalized function including the sum of: (1) the error signal times a proportional gain factor ("P-gain") and (2) the integral of the error signal times an integral gain factor ("I-gain"). By adjusting the P and I-gain factors, a wide variety of transfer functions may be affected, when combined with the physical transfer function of the motor system or process, to produce the desired system response.
Selecting the proper P and I-gain factors to produce a desired system response has been the subject of considerable study. If the transfer function of the physical system to be controlled is well known and may be approximated by a linear system, the appropriate P and I-gain factors may be calculated according to desired tradeoffs by a number of well known methods. More typically, however, the precise transfer characteristics of the physical system are not well known and/or are non-linear. In these cases, the proper gain factors must be approximated, typically by a human expert applying "rules of thumb". Ideally, P and I-gain factors are chosen such that tension is instantaneously controllable.
Unfortunately, while tension regulation appears to be a simple task with a tension feedback loop, time varying roll inertia complicates tension control. Roll inertia is the momentum associated with a spool and material accumulated thereon. Inertia depends on a number of different factors. First, inertia depends on the quantity of accumulated material on a spool. When there is little or no material on the spool, inertia is relatively small. However, when a spool is essentially fully loaded inertia is relatively large.
Second, inertia depends on material density. High density materials are heavier than low density materials. Therefore, given a specific roll radius and spindle speed, a high density material roll has more inertia than a low density material roll.
Third, inertia depends on spindle characteristics. Just as different materials and different accumulated material amounts affect roll inertia, spindle size and construction also affect roll inertia. A relatively heavy spindle has more inertia than a lighter spindle. A spindle with mass distributed near its circumference has more inertia than a similarly sized spindle with a more centrally concentrated mass.
Inertia directly impacts the effectiveness of a torque in modifying spindle speed. For example, when the quantity of material accumulated on a spool is small and roll inertia is small, torque required to modify spindle speed is relatively small. However, when the quantity of material accumulated on a spool is large and roll inertia is large, torque required to modify spindle speed is relatively large.
Inertia's effect on tension correction depends on the level or degree of correction required. For example, clearly given a finite acceleration period, torque required to overcome inertia and accelerate a spindle 2 p.u. within the acceleration period is greater than the torque required to overcome inertia and accelerate the spindle 1 p.u.
In addition, torque required to overcome inertia during acceleration and deceleration is affected by mechanical system friction collectively referred to herein as spindle friction. Spindle friction tends to reduce spindle speed. For this reason, friction cooperates with a decelerating torque to decelerate a spindle while it acts against an accelerating torque. For control purposes, spindle friction can be lumped together with inertia. When friction and inertia are lumped together, friction in conjunction with inertia results in an overall reduced inertia (aiding deceleration) during deceleration and an overall increased inertia (impeding acceleration) during acceleration.
Because of inertia, typical PI regulators cannot operate quickly enough to compensate for tension errors.
To compensate for the effects of inertia, many winder motor controllers have been equipped with an inertia compensator. Inertia compensators compensate for system nuances and material characteristics by providing a torque correction to be added to the torque signal provided by the PI regulator. The resulting modified torque signal is used to drive the winder motor. To this end, the compensator is provided with various material specific constants indicating material characteristics, receives several feedback signals indicating instantaneous system characteristics and uses the constants and signals to generate the torque correction signal. The constants often include material density, material width (i.e. the width of a material roll), a spindle inertia constant indicating inertia associated solely with an empty spindle and acceleration and deceleration "kicker" signals indicating how the correction torque should be modified to account for spindle acceleration and deceleration. The feedback signals often include material line speed and roll diameter.
While inertia compensators work well to compensate for roll inertia and friction, they have a number of shortcomings. First, many winders are used to wind several different types of material at different times. Where materials to be wound have different characteristics, during a commissioning procedure, a winder operator must identify material characteristics and program the compensator to compensate for the characteristics. In addition, spindle inertia must be derived and the acceleration and deceleration kickers have to be adjusted when spindles are changed. Moreover, P and I-gain factors have to be determined for new spindle and material combinations. While identifying material and system characteristics may not be extremely burdensome where material and spindle changes are irregular, in many industries changes are routine and characteristic identification and controller programming become tedious.
Second, often material characteristics are not precisely known. For example, with sheet steel there are many different grades of material, each grade having a material density within a steel density range. In the case of steel, instead of knowing the exact density of sheet steel, an operator typically selects a density from within the density range which is then used by the inertia compensator for tension control. While the selected density may fortuitously be accurate, often the selected value will have some error. In this case, tension control may be imperfect.
Third, even after a material characteristic is determined, the material provided by a manufacturing machine for winding may change slightly such that the determined characteristic is no longer valid. For example, even after density is correctly determined and during winding, density of material provided may change slightly. Where density change is not accounted for, the inertia compensator can generate an incorrect torque correction thereby causing an incorrect material tension.
Therefore, it would be advantageous to have a controller which receives standard winder feedback signals, uses those signals to automatically determine material characteristics and operating characteristics and which can automatically adjust motor operation based on the automatically determined characteristics to affect a desired tension without requiring a prolonged and tedious commissioning procedure.